


The Beginning: The Light in the Dark

by Jdragon122



Category: Supernatural
Genre: Alternate Universe - Merpeople, Angst and Hurt/Comfort, Cas is the best ocean pancake, Dean doesn't like the open ocean, Desperate Dean Winchester, Eel!Dean, Eel!Sam, First Meetings, M/M, Manta ray!Cas, MerMay, Merperson Castiel (Supernatural), Merperson Dean Winchester, Merperson Sam Winchester, Poisoned Sam Winchester, cecailia, mermaid au, my friends have dubbed him the flap flap of the sea
Language: English
Status: Completed
Published: 2020-05-01
Updated: 2020-05-01
Packaged: 2021-03-02 01:55:20
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,179
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23937232
Author URL: https://archiveofourown.org/users/Jdragon122/pseuds/Jdragon122
Summary: Sam and Dean have always lived in the warm corals of home. But when they’re attacked by a group of cecailia, Sam falls ill from their poison, forcing Dean to venture out in search of an antidote. But the ocean is vast and dark, he might need a little light to guide his way. Will he cure Sam in time? And what help will he meet along the way?
Relationships: Castiel/Dean Winchester
Comments: 16
Kudos: 62
Collections: Best Mermaid Fanfiction





	The Beginning: The Light in the Dark

**Author's Note:**

> Hey guys! Been awhile since I've written hasn't it? If you know me, you know I love my fantasy AUs - and my friends over at [DestielArtistsUnited](https://destielartistsunited.tumblr.com/) and my bestest bestie and fellow creator [Lunastories](https://lunastories.tumblr.com/) have only encouraged me even more! They're the ones that introduced me to the idea of manta mermaids! SO of course I had to make that into a Destiel AU. I'd also like to thank my lovely beta [Queen0fHearts](https://archiveofourown.org/users/Queen0fHearts/pseuds/Queen0fHearts) for being so kind as to look over this for me.
> 
> This is originally based off of [this](https://jdragon122.tumblr.com/post/615159545635930112/the-folks-at-destielartistsunited-introduced-me) art of mine, followed by [this](https://jdragon122.tumblr.com/post/615261287988035584/manta-mercas-looked-lonely-so-i-gave-him-eeldean) one! I thought it only fair they get a story of how they met. I hope you guys enjoy this short one shot! <3

Dean had never been this far offshore before. The dark depths below him yawn wide, twisting his stomach into a knot. He never realized how much he enjoys the warm, welcoming softness of the sand. Out here in the open ocean, there is nothing but cold expanse ahead of him and a gaping void below him. It sends a shiver down his tail.

He never wanted to come out here, but it isn’t as if he has much of a choice. Fighting off the cecaelia who’d invaded their reef had done a number on him. And Sam… the big lug _had_ to get nicked by one of their poisonous spears — and of course the only antidote is several miles out to sea in a vague location on the ocean floor. At least that’s what he was able to coax from the remaining cecelia before dispatching them.

Swim towards the moon until you find the glow, they’d said. Dean hates the vagueness of that statement and he knows it very well could be bullshit meant to get him killed. But if he doesn’t try then Sam will die and there isn’t much for Dean to live for anyway after that.

Dean continues to swim valiantly forward, long green-spotted tail curling from side to side. His nocturnal vision has done him well enough so far. He’d seen a few fish pass by in the dark, but he still loathes the emptiness that surrounds him; a great emptiness that would swallow him whole and no one would even hear him scream.

He breaches the surface every once in a while to make sure he’s still following the moon. He isn’t sure how much time has passed but he knows it’s been a while. Even the moon has begun to fade and the sun’s light is slowly peeking out from below the horizon. Dean’s exhausted; he’s been swimming all night with no rest, but he keeps going. He hopes to the gods that the cecaelia hadn’t lied. It’s a long shot by far but he needs this. Sam needs this.

His movements gradually begin to slow. He can feel the exhaustion weighing him down in the water. This is it, isn’t it? It’s all been a lie and now he’s stranded in the middle of nowhere. But he couldn’t give up, not yet, even if his belly protests with hunger and his limbs sink like stones.

As the sun finally rises from its ocean bed, Dean stops. He watches above the waves as the sun lights up the water with a myriad of white reflections. It’s beautiful and yet, Dean can’t let the morning calm him as it so often did. “I’m sorry, Sammy,” he pants, tears coming to his eyes, “I failed you.” He stares for a while, catching his breath and trying to come to grips with what is to come. There’s no glow and there’s no antidote. Sam is going to die.

The rebellious spirit within Dean refuses. Sam would fight the poison and he would win, but it’s only a fleeting hope. He should turn back, spend as much time with his brother as he can.

But anger flares within his fighting soul. He disappears back into the depths, diving for the seafloor. Maybe he’d missed it, maybe he wasn’t looking carefully enough. Darker and darker the sea becomes as he abandons the new light of day. He doesn’t want it if Sam won’t see it either.

Several minutes pass before he is forced to stop, his limbs refusing to let him go any further. He yells in frustration, his lone voice echoing into the water and out into the unknown. He hates this place and its infinite nothingness. He wants home, he wants Sam. For a while, he floats in silence and then he hears an answering call.

Dean blinks, wondering if it’s just the exhaustion getting to him. Tentatively, he calls out again, this time a series of questioning notes. To his disbelief, he sees a glow.

It’s small at first, a single pinpoint of light growing larger and brighter as it approaches. Dean’s eyes widen as he realizes that it’s joined by many, _many_ more lights. He stares in wonder as the darkness suddenly begins to twinkle like the night sky, stars carpeting the vastness below him.

Dean can do nothing but stare in awe, it’s like nothing he’s ever seen. They blink in and out, pulsing to the push and pull of the current. Swimming closer he finally makes out the shape of the creatures decorating the ocean deeps. Manta rays, hundreds of them. The tips of their fins glow with bioluminescent light, each with different patterns and hues that blend together into one great mass. Their glow fades in and out as they lazily swim along, flying on the current like a flock of seabirds. Dean can make out some faces in the mix, most of them not bothering him with more than a glance. There’s so many of them that Dean starts to go cross-eyed trying to pick out individuals. They’re all so fluid, a continuous moving string of light, never slowing or stopping. Then he sees those eyes.

They’re the only solid thing in the cloud of fins, frozen in place among the crowd. As the rest of the swarm moves, those eyes stay in place, watching him — bright, blue, and piercing his very soul. Dean can’t look away.

The eyes rise, breaking through the veil of fins to reveal the manta’s full form. He ascends, barely needing to move as he rises in front of Dean. Dean stares at the wide expanse of his deep blue fins, large and powerful. His underbelly glitters with more light, swirling around his form and defining his upper torso. Dark hair sways in the current and deep blue claws decorate his fingertips. Dean can’t speak, too hypnotized to do anything but look.

“Uh, hello,” the voice that reaches him is gravelly and deep. If it wasn’t for the awkward greeting he might’ve swooned.

“Um, hi,” he replies, voice not as strong as he would like.

The manta stays uncomfortably silent for a moment before continuing, “Was that you we heard? You seemed to be in some trouble.”

Dean swallows, not sure why he’s so nervous, “Yeah I… I’ve been swimming all night trying to find something.” The manta tilts his head, curiosity clear in his eyes. “It’s my brother. He was poisoned by a cecaelia blade and I’m trying to find an antidote.”

A look of understanding and sympathy crosses the manta’s face. “I’m sorry to hear that. You clearly care very much for your brother.” Dean can do nothing but nod, a lump forming in his throat.

The manta looks down thoughtfully, his brow furrowing. “May I ask what color is his wound?”

Dean blinks in surprise, “Uh… yeah. Black.”

The manta nods, “I know the venom they used, it is very typical for cecaelia. You say you don’t have an antidote?” Dean shakes his head. He looks Dean up and down for a moment, making him a little self-conscious. “You’re an eel. You live very far from here in the corals, I presume.” Dean nods once more. “Then we could help you. There should be ingredients for an antidote already there —”

“Castiel,” a stern voice calls from behind Dean. He turns to see a larger, dark manta approach. He doesn’t try to hide his disdain for Dean as he looks him up and down before turning to his fellow manta.

Castiel. Dean repeats the name in his head. It’s an odd name but rightly fitting, grand and elegant.

“Uriel,” Castiel nods in greeting, voice slightly edged with annoyance.

“What do you think you’re doing, Castiel?”

“I’m offering our help.”

“ _We_ have no business with him, leave him to his fate.”

“But we will already be passing by the corals. It will be of no hindrance to us to help him.”

Uriel sneers, “We still have many miles to swim, unless you forgot how long it takes to migrate.

Castiel grits his teeth, “No. I have not.”

“Good,” Uriel turns to swim away, “Then let us continue.”

Dean’s hopes sink as he sees Castiel start to follow Uriel. He stares at Castiel, his eyes wide and pleading but to his surprise, a small rebellious smirk pulls at Castiel’s lips. As the manta swims past he grabs his shoulder, pulling him along.

“Castiel!” Uriel bellows as Castiel continues to take Dean with him.

Castiel cuts him short, “You said _we_ won’t help him, you never said _I_ couldn’t.”

Uriel’s mouth twitches, anger tensing his shoulders. He looks like he wants to argue but instead, he says, “Fine. But if you fall behind we won’t wait for you.” And with that, Uriel disappears back into the swarm.

Dean stares in disbelief. He feels the tension leave his body and relief flood him. He turns his eyes to Castiel, the manta’s hand still warm on his shoulder. The manta is looking at him with a soft smile, it’s so enduring and genuine that Dean might cry.

“Thank you,” his voice cracks as he tries to hold back just how grateful he feels. No one but Sam has done something so kind for him. To Castiel, Dean is a stranger and still, he’s doing this for him.

“You’re welcome,” Castiel’s voice fills Dean with warmth. “Come now, you can swim with me.”

Dean wordlessly follows the manta into the current of fins and bodies. Their eyes are immediately on Dean, some annoyed, others impartial but it makes Dean realize just how starkly he stands out against the mantas. His spotted green hide and long tail are odd additions to the mix of blues and dark browns.

“Don’t worry,” Castiel whispers to him, “They just look grumpy but they won’t lay a claw on you while I’m here.” At that, the warm feeling in Dean’s chest only grows. Castiel eventually slips his hand off of his shoulder and Dean almost protests but thinks better of it. They swim wordlessly side by side.

After a while, Dean feels himself slow once more, having a hard time keeping up with the mantas around him, forcing him to swim in bursts to stay fin to fin with Castiel. The manta eyes him worriedly but says nothing until Dean accidentally bumps into one of the adjacent mantas.

He gives them and Castiel an apologetic smile. Castiel just sighs and, to Dean’s surprise, takes both of his hands. Wordlessly, Dean lets the manta drape his arms over the manta’s shoulders, clasping their hand together on his chest so that Dean is laying atop Castiel’s back, drifting along with him with no effort. Usually, Dean would protest for his dignity’s sake but as soon as he relaxes on Castiel’s back he forgets all about it. He’s so tired and hungry, and Castiel is warm and soft. He sighs softly and lets his eyes close.

“Thank you, again,” he manages to mutter.

“Of course.” Castiel’s voice rumbles in his chest and hums against Dean’s skin. He hums, falling quickly towards sleep.

“Before you drift off,” Castiel chuckles beneath him, “May I ask your name?”

Of course! How could Dean not even have introduced himself yet? Stupid!

“Dean, my name is Dean Winchester.”

Castiel hums, “‘Nice to meet you, Dean. I’m Castiel.”

“Nice to meet you too, Cas,” Dean smiles sleepily.

“Close enough,” Cas smiles.

It’s all so nice, Cas’s hands on his, the slow movement of his fins. The gentle cycle continues to lull him further and further. It feels like forever since he’d slept.

The last thing Dean hears before he succumbs is a gentle, “Sleep well, Dean.”

_________________

Dean awakes to a gentle tug on his arm. “Hm, what is it?” he murmurs.

“We’re here, Dean.”

Dean’s eyes fly open. For a second, he forgets that he isn’t home in his sand bed. “Home?” he asks, trying to blink himself awake.

“Yes, home,” Cas replies. He pauses, then adds with a somber tone, “You may want to get up soon, I can only stay for so long without falling behind. And it’s been several hours, your brother may not have long.”

At that, Dean is wide awake. The pressure around his hands cease and he immediately rolls back into the water on his own, rubbing at his eyes to take in the world around him. They aren’t with the mantas anymore, he turns to see the swarm of them just offshore, a dark mass streaking across the ocean. They were certainly back at the corals.

“You look for your brother and call to me when you’ve found him. I’m going to go prepare the antidote.”

“Okay,” Dean doesn’t question him. Instead, he gives him a nod then shoots off to search the reef. Although having awoken much more rested, Dean can’t help but feel tired again at the thought that he may be too late. But he’s not, he can’t be. He swims through the reef, yelling Sam’s name as he goes. There’s no response, making Dean’s panic rise.

“Sam!” he calls out, checking all of their favorite spots and hideaways, the coral cave, the shallows, the sponge beds. He looks everywhere until he finally sees him.

“Sam!!” he spots his brother lying on the sand, hidden by a canopy of coral and sponges. Dean’s heart jumps to his throat, rushing to Sam’s side. He shakes him hard. “Wake up, you son of a bitch.” His brother’s face is a pale yellow, scales dull, and limbs limp. Dean puts a hand to his gills and holds his breath.

For a second he expects the worst, but then he feels a rush of water. “Oh my god,” he sighs in relief. Sam was still alive. “Cas!” he calls, “I found him!”

In a second, Dean sees a large shadow fall over him and looks up to see Cas diving towards him, something in his hand.

Dean respectfully moves to make room for the manta, knowing now is not the time to ask questions. He swallows, his heart beating rapidly as he watches Cas work. The manta rolls his brother onto his back, observing the wound on his chest. He gently takes the shell in his hand and scoops out a goopy substance. Carefully, he applies it to the wound, covering it thoroughly in a green paste. He sits back for a moment, then looks to Dean apologetically. “He needs to be awake for this next part.” Dean frowns in confusion then watches as Cas whips out his stinger and jabs Sam with a small current of electricity.

Sam flinches awake with a squeal, eyes flying open. Dean would’ve laughed if he wasn’t so relieved. “What the fuck!” Sam cries out, and oh how glad Dean is to hear his voice.

“Sammy! I’m right here, okay,” he puts a hand on his shoulder, “I’ll explain everything later but right now you need to do exactly what he says, got it?” Sam’s eyes look glazed and confused as he glances to Cas but one look at Dean and the questions die on his tongue.

“Okay,” he says weakly. Dean looks to Castiel who’s waiting patiently and nods.

Castiel immediately takes the handful of algae and coral he’d gathered and grinds them together in his hands. He gingerly leans over to Sam. “You need to eat this.”

Sam nods, “Okay.” With a shaky hand, Sam takes the strange mix and chews it. He grimaces, “Gah, this tastes like algae.”

“It is algae,” Cas deadpans. Dean’s got to give it to the guy, he has a sense of humor.

Once Sam has swallowed, they both look to Cas expectantly.

“And?” Dean asks, “What’s next?”

Cas relaxes, “That’s it. All he can do now is rest and let the medicine do its work.”

Dean sighs in relief. It was all okay now, Sammy was saved. He looks down at his little brother who smiles weakly back. Fuck it. He grabs him in a hug.

Sam feebly returns the favor, wrapping his too-long arms around him. “Thank you,” Sam says, “Thank you both.”

At that, Dean lets go. He looks to Cas who is watching him with a hint of sadness.

“You saved us, both of us,” Dean says with a smile. God knows Cas deserves it. “We’re strangers to you and still you helped. We can’t thank you enough. _I_ can’t thank you enough.”

A slight blush reddens Cas’s cheeks. “You’re welcome. I was happy to help. There’s no need for pointless suffering if you can do something about it.”

Dean’s chest swells with pride for the manta. He’d just met this man, but he knew that there was something special about him. He wanted to know more, to explore who he was and bask in his presence.

But he couldn’t.

The reminder of that reality makes Dean’s heart sink. His face falls when it clicks that Cas couldn’t stay. Castiel must read his expression because his eyes reflect what Dean feels. Dean swallows, “You need to leave now, don’t you?” Castiel nods.

Dean looks back to the other mantas, realizing they were almost completely hidden behind the bend of the coast now.

He didn’t know what to say. His throat tightened painfully as words refused to come out. How would he say goodbye? How cruel was it, to have just met someone he felt _something_ for only to have them leave forever?

Castiel starts for him, the manta rising from his place in the sand, “It was nice to meet you, Dean Winchester. May the sea bring you many blessings and —”

“Will I ever see you again?” Dean bursts out, unable to hold back.

Castiel’s mouth hangs open mid-sentence, unsure how to answer.

“You’ll migrate back this way next season right?” Dean tries to reason, “You could stop by then before going north?”

Cas’s eyes light up in surprise, “I hadn’t thought of that.” The slightest of smiles pull at his lips, “Yes, Dean. Then I think I will see you again.”

The warmth flares brighter in him now, chasing away the emptiness that had been peeking through.

“I can’t wait,” Dean’s smile wrinkles the corner of his eyes. “So, I guess I’ll see you around.”

Cas smiles wider now, “Yes, I’ll see you around.” The manta glances back to where his kind has now almost faded. He looks to Dean one more time, raising his hand to wave. “Till next time?”

Dean waves back, “Till next time.”

Satisfied, Castiel turns away and swims off to join his fellow mantas. Dean watches him go, his form growing smaller in the distance until he melds with the ocean. He rounds the corner of the coast and is gone.

Dean takes a deep breath and sighs. It’d be a long while till the next migration, but Dean is willing to wait to see Castiel again.

He turns his gaze away only to stop at the odd look Sam is giving him. “What?” he frowns.

Sam smirks, “Nothing. Absolutely nothing at all.”

**Author's Note:**

> And that's how it starts folks! I'm thinking about making some other short stories with them but I can't make any guarantees. But I hope you guys liked this short introduction of these two! <3 tell me what you think! until next time lovelies!


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